The present invention relates to novel membranes and to a process for their production.
Zeolites and related crystalline materials are well known for their ability to accomplish separations and to act as catalysts. Much prior art describes the preparation of supported zeolite catalysts: many different types of zeolite have been deposited on many different types of support. U.S. Pat. No. 3,244,643 describes a method "for utilising as a support substances which ordinarily would be difficult to composite with the crystalline aluminosilicate into hard particles by conventional pelleting or extruding operations". This aim is accomplished by impregnating a porous, cage-like, solid support with zeolite synthesis gel under conditions that crystals of the zeolite grow within the pores of the support to a size sufficiently large that they are retained within the pore structure of the support but not to a size exceeding the effective diameter of the pores. The exemplified procedure describes the impregnation of particulate supports with the zeolite gel under vacuum.
U.S. Pat. No. 3,730,910 describes the formation of zeolite surfaces on substrates, the surfaces essentially having the same shape and form as the substrate. These shaped materials may be used as catalysts or adsorbents. There is no suggestion that they could form membranes.
U.S. Pat. No. 4,578,372 describes a process for preparing magnetizable catalytically active particles which comprises depositing a layer of zeolite on the surface of magnetizable particles.
Membranes are well known in separation applications, and membranes incorporating zeolites are known. Applied Catalysis 49 (1989) 1-25 is a review of catalysis with permselective inorganic membranes.
Many documents describe membranes in which zeolite crystals are bound to the surface of a porous substrate using a "glue" or binder. Commonly, this takes the form of a layer of amorphous material between the surface and the zeolite. The following documents describe membranes of this type:
(i) DE-A-3827049. This document describes a system comprising a support of micro-porous material with pore sizes less than the mean particle size of the zeolite crystals, which is provided with a "nucleo-active" surface populated by a layer of zeolite crystals bridging the pore openings. This "nucleo-active surface" results in a layer of amorphous material between the surface of the support; this layer is clearly shown in the drawings of the specification. PA1 (ii) CA 1235684 claims a filter for substance separation, comprising a substrate made of a porous glass and a zeolite-based film formed directly on the porous glass, the zeolite-based film having a thickness of 1 micron to 500 micron. The filter is prepared by suspending, for example, a borosilicate glass in an aqueous solution of sodium hydroxide and tetrapropylammonium bromide, and heating in an autoclave. The results of this method of synthesis are shown in Table 2 of the specification. The resulting membrane has a distinct layer of amorphous material between the surface of the support and the zeolite layer. We have reproduced these results, and shown that it is not possible to produce a membrane without a layer of amorphous material using the instructions in the specification. PA1 (iii) JP-A-63287504 describes a separation membrane in which a hydrophobic thin film or a thin film containing a hydrophobic substance is adhered to one surface of a porous inorganic support. The hydrophobic substance can include silicalite or zeolite. As with the documents discussed above, a layer of binder material is used to adhere the crystals to the surface. This is described in Example 3 of the specification, where an alumina sol is deposited on the support surface prior to deposition of the zeolite crystals. PA1 (iv) JP-A-63291809 describes a membrane comprising a film of zeolite on a porous alumina carrier. We have attempted to reproduce the preparation of membranes detailed in the specification, but without success. PA1 (v) EP-A-180200 describes a membrane comprising a porous support impregnated with fine particles of a zeolite. The membrane is prepared by permeating, for example, an ultrafiltration membrane, or a porous glass, with an alkaline solution of ultrafine (e.g. less than 75 angstrom diameter) zeolite particles. These particles become lodged in the pores of the support, where they are "glued" in place using amorphous material.
EP-A-180200 makes reference to earlier EP-A-135069 (EP application no. 84108871.9) from the same inventor, H. Suzuki. This document describes a process for preparing a zeolite membrane in which a zeolite synthesis gel is passed through a microfilter and deposited in a thin film on the surface of a support. In his later document, Suzuki states of the membrane of his earlier document that the zeolite membrane was " . . . easy peeling, stripping off and difficult to use". Our attempts to produce membranes by repeating Suzuki's instructions have been unsuccessful.
Small-scale zeolite membranes can be bought from Suzuki. Such membranes do not have a continuous coverage of zeolite, and are too fragile to use in any large scale application.
In summary, although there has been extensive research in the field of zeolite membranes, there is no disclosure in the prior art by which a zeolite membrane having a continuous layer of zeolite directly connected to the surface of a support, can be prepared.